This invention relates to a process for removing ammonia from organic solutions of an oxime-metal chelate prior to treating the solution to remove the metal therefrom.
In U.S. Pat. No. 3,853,725, entitled "Selective Stripping Process"; U.S. Pat. No. 3,855,090, entitled "Process for Recovering Nickel Selectively"; and, copending application Ser. No. 266,986, entitled "Selective Solvent Extraction Process"; all filed on June 28, 1972, various processes are described for recovering nickel or copper from aqueous ammoniacal leach solutions. In these processes, an ore containing copper and/or nickel is contacted with an aqueous ammoniacal solution, generally ammonia-ammonium carbonate having a pH above about 9, to form an aqueous solution of these metals. The copper and/or nickel contained in the aqueous metal-rich solution is chelated with an oxime in a solvent extraction step. The chelate is stripped with an aqueous acid solution to transfer the metal from the oxime to the acid solution; and, the metal-rich stripping solutions are treated by electrowinning to recover pure nickel and copper.
In one process, separation of nickel and copper is achieved in the solvent extraction step. In an alternative process, selective separation of nickel and copper is affected in a stripping step following coextraction of nickel and copper from an aqueous ammoniacal solution with an organic oxime solution. In this latter process, which is shown schematically in FIG. 1, in the nickel strip stage 10, hydrogen ion in the aqueous solution is exchanged selectively for nickel ion in the oxime. A copper-rich oxime solution recovered from this acid nickel stripping stage is contacted with an aqueous acid solution in a separate copper stripping stage 12 wherein the copper ion in the oxime solution and hydrogen ion in the aqueous acid solution are exchanged. The nickel-rich aqueous solution and the copper-rich aqueous solution, recovered from each of these stripping steps, are then treated by electrowinning in separate steps to obtain pure nickel and copper metal.
When oximes are used to extract metal values from ammoniacal leach solutions, some ammonia is transferred from the original ammoniacal leach solution to the oxime. Furthermore, essentially all of the ammonia extracted by the oxime is transferred to the acid aqueous stripping solutions. This transfer results in substantial reagent losses and formation of ammonium salts which must be removed from the process. The formation of the ammonium salts in the stripping step is particularly undesirable in the case of nickel stripping since it results in the precipitation of nickel as NiSO.sub.4 .(NH.sub.4).sub.2 SO.sub.4. Indeed, in order to render extraction processes attractive, from a commercial standpoint, it is necessary that any ammonia be removed from the organic oxime solution prior to the stripping steps.
In U.S. patent application Ser. No. 317,003 now abandoned in favor of Ser. No. 610,935, entitled "Process for Removing Ammonia from Solutions of Oxime-Metal Chelates" by Skarbo et al, filed on Dec. 20, 1972, the teachings of which are incorporated herein by reference, a process for removing ammonia from oxime-metal chelates is disclosed. The process comprises countercurrently contacting the organic oxime with an aqueous solution stream 11 containing an agent such as ammonium carbonate, ammonium bicarbonate, alkaline metal bicarbonate or mixtures of the foregoing agents in a first scrub stage 14. As a result of contacting the oxime, which may contain extracted ammonia, with the foregoing agent in the primary scrub stage 14, a large portion of the extracted ammonia is removed from the oxime. The ammonia-depleted organic oxime leaving primary scrub stage 14 can be treated in the secondary scrub stage 16 to remove additional ammonia prior to recovering metals therefrom. A typical method of removing residual ammonia in the secondary scrub stage 16, is to contact the oxime with an aqueous solution of sulfuric acid. In secondary scrub stage 16 of the prior art procedure, a recycled solution stream 18 containing 100 g/l (NH.sub.4).sub.2 SO.sub.4 and H.sub.2 SO.sub.4 at a pH of 3-4 is used to scrub the last 50-100 ppm NH.sub.3 from the metal loaded organic phase. At an O/A ratio = 1.0, this amount of NH.sub.3 scrubbed from the organic is enough to neutralize the acid in the aqueous scrub solution causing the pH to rise from 3-4 up to .apprxeq. 6. Consequently, continuous addition of H.sub.2 SO.sub.4 to maintain a pH of 3-4 in the recycle scrub solution 18 is required. Thus: ##STR1##